Making proper_table_name take in options.

The options will specify the prefix and the suffix. Also, I'm moving the method to be an instance method on the `Migration` instance. This makes more sense than being a class method on the `Migrator` class because the only
place that uses it is on a `Migration` instance (in a method_missing hook). The logic for the `Migrator` shouldn't be doing any work to calculate the table name, it should be the `Migration` itself.

Also made some small indentation fixes.

This will hopefully make it easier to get rid of the call to `ActiveRecord::Base` by making the options explicit.

\cc @spastorino